With regard to sleep and waking, we must consider what they are: whether
they are peculiar to soul or to body, or common to both; and if common,
to what part of soul or body they appertain: further, from what cause it
arises that they are attributes of animals, and whether all animals share
in them both, or some partake of the one only, others of the other only,
or some partake of neither and some of both.

Further, in addition to these questions, we must also inquire what
the dream is, and from what cause sleepers sometimes dream, and sometimes
do not; or whether the truth is that sleepers always dream but do not always
remember (their dream); and if this occurs, what its explanation
is.

Again, [we must inquire] whether it is possible or not to foresee
the future (in dreams), and if it be possible, in what manner; further,
whether, supposing it possible, it extends only to things to be accomplished
by the agency of Man, or to those also of which the cause lies in supra-human
agency, and which result from the workings of Nature, or of
Spontaneity.

First, then, this much is clear, that waking and sleep appertain
to the same part of an animal, inasmuch as they are opposites, and sleep
is evidently a privation of waking. For contraries, in natural as well
as in all other matters, are seen always to present themselves in the same
subject, and to be affections of the same: examples are-health and sickness,
beauty and ugliness, strength and weakness, sight and blindness, hearing
and deafness. This is also clear from the following considerations. The
criterion by which we know the waking person to be awake is identical with
that by which we know the sleeper to be asleep; for we assume that one
who is exercising sense-perception is awake, and that every one who is
awake perceives either some external movement or else some movement in
his own consciousness. If waking, then, consists in nothing else than the
exercise of sense-perception, the inference is clear, that the organ, in
virtue of which animals perceive, is that by which they wake, when they
are awake, or sleep, when they are awake, or sleep, when they are
asleep.

But since the exercise of sense-perception does not belong to soul
or body exclusively, then (since the subject of actuality is in every case
identical with that of potentiality, and what is called sense-perception,
as actuality, is a movement of the soul through the body) it is clear that
its affection is not an affection of soul exclusively, and that a soulless
body has not the potentiality of perception. [Thus sleep and waking are
not attributes of pure intelligence, on the one hand, or of inanimate bodies,
on the other.]

Now, whereas we have already elsewhere distinguished what are called
the parts of the soul, and whereas the nutrient is, in all living bodies,
capable of existing without the other parts, while none of the others can
exist without the nutrient; it is clear that sleep and waking are not affections
of such living things as partake only of growth and decay, e.g. not of
plants, because these have not the faculty of sense-perception, whether
or not this be capable of separate existence; in its potentiality, indeed,
and in its relationships, it is separable.

Likewise it is clear that [of those which either sleep or wake]
there is no animal which is always awake or always asleep, but that both
these affections belong [alternately] to the same animals. For if there
be an animal not endued with sense-perception, it is impossible that this
should either sleep or wake; since both these are affections of the activity
of the primary faculty of sense-perception. But it is equally impossible
also that either of these two affections should perpetually attach itself
to the same animal, e.g. that some species of animal should be always asleep
or always awake, without intermission; for all organs which have a natural
function must lose power when they work beyond the natural time-limit of
their working period; for instance, the eyes [must lose power] from too
long continuedseeing, and must give it up; and so it is withthe hand and every other
member which has a function. Now, if sense-perception is the function of
a special organ, this also, if it continues perceiving beyond the appointed
time-limit of its continuous working period, will lose its power, and will
do its work no longer. Accordingly, if the waking period is determined
by this fact, that in it sense-perception is free; if in the case of some
contraries one of the two must be present, while in the case of others
this is not necessary; if waking is the contrary of sleeping, and one of
these two must be present to every animal: it must follow that the state
of sleeping is necessary. Finally, if such affection is Sleep, and this
is a state of powerlessness arising from excess of waking, and excess of
waking is in its origin sometimes morbid, sometimes not, so that the powerlessness
or dissolution of activity will be so or not; it is inevitable that every
creature which wakes must also be capable of sleeping, since it is impossible
that it should continue actualizing its powers perpetually.

So, also, it is impossible for any animal to continue always sleeping.
For sleep is an affection of the organ of sense-perception--a sort of tie
or inhibition of function imposed on it, so that every creature that sleeps
must needs have the organ of sense-perception. Now, that alone which is
capable of sense-perception in actuality has the faculty of sense-perception;
but to realize this faculty, in the proper and unqualified sense, is impossible
while one is asleep. All sleep, therefore, must be susceptible of awakening.
Accordingly, almost all other animals are clearly observed to partake in
sleep, whether they are aquatic, aerial, or terrestrial, since fishes of
all kinds, and molluscs, as well as all others which have eyes, have been
seen sleeping. 'Hard-eyed' creatures and insects manifestly assume the
posture of sleep; but the sleep of all such creatures is of brief duration,
so that often it might well baffle one's observation to decide whether
they sleep or not. Of testaceous animals, on the contrary, no direct sensible
evidence is as yet forthcoming to determine whether they sleep, but if
the above reasoning be convincing to any one, he who follows it will admit
this [viz. that they do so.]

That, therefore, all animals sleep may be gathered from these considerations.
For an animal is defined as such by its possessing sense-perception; and
we assert that sleep is, in a certain way, an inhibition of function, or,
as it were, a tie, imposed on sense-perception, while its loosening or
remission constitutes the being awake. But no plant can partake in either
of these affections, for without sense-perception there is neither sleeping
nor waking. But creatures which have sense-perception have likewise the
feeling of pain and pleasure, while those which have these have appetite
as well; but plants have none of these affections. A mark of this is that
the nutrient part does its own work better when (the animal) is asleep
than when it is awake. Nutrition and growth are then especially promoted,
a fact which implies that creatures do not need sense-perception to assist
these processes.

Part 2

We must now proceed to inquire into the cause why one sleeps and
wakes, and into the particular nature of the sense-perception, or sense-perceptions,
if there be several, on which these affections depend. Since, then, some
animals possess all the modes of sense-perception, and some not all, not,
for example, sight, while all possess touch and taste, except such animals
as are imperfectly developed, a class of which we have already treated
in our work on the soul; and since an animal when asleep is unable to exercise,
in the simple sense any particular sensory faculty whatever, it follows
that in the state called sleep the same affection must extend to all the
special senses; because, if it attaches itself to one of them but not to
another, then an animal while asleep may perceive with the latter; but
this is impossible.

Now, since every sense has something peculiar, and also something
common; peculiar, as, e.g. seeing is to the sense of sight, hearing to
the auditory sense, and so on with the other senses severally; while all
are accompanied by a common power, in virtue whereof a person perceives
that he sees or hears (for, assuredly, it is not by the special sense of
sight that one sees that he sees; and it is not by mere taste, or sight,
or both together that one discerns, and has the faculty of discerning,
that sweet things are different from white things, but by a faculty connected
in common with all the organs of sense; for there is one sensory function,
and the controlling sensory faculty is one, though differing as a faculty
of perception in relation to each genus of sensibles, e.g. sound or colour);
and since this [common sensory activity] subsists in association chiefly
with the faculty of touch (for this can exist apart from all the other
organs of sense, but none of them can exist apart from it-a subject of
which we have treated in our speculations concerning the Soul); it is therefore
evident that waking and sleeping are an affection of this common
and controlling organ of sense-perception. This explainswhy they belong to all animals, for touch [with which this
common organ is chiefly connected], alone, [is common] to all
[animals].

For if sleeping were caused by the special senses having each and
all undergone some affection, it would be strange that these senses, for
which it is neither necessary nor in a manner possible to realize their
powers simultaneously, should necessarily all go idle and become motionless
simultaneously. For the contrary experience, viz. that they should not
go to rest altogether, would have been more reasonably anticipated. But,
according to the explanation just given, all is quite clear regarding those
also. For, when the sense organ which controls all the others, and to which
all the others are tributary, has been in some way affected, that these
others should be all affected at the same time is inevitable, whereas,
if one of the tributaries becomes powerless, that the controlling organ
should also become powerless need in no wise follow.

It is indeed evident from many considerations that sleep does not
consist in the mere fact that the special senses do not function or that
one does not employ them; and that it does not consist merely in an inability
to exercise the sense-perceptions; for such is what happens in cases of
swooning. A swoon means just such impotence of perception, and certain
other cases of unconsciousness also are of this nature. Moreover, persons
who have the bloodvessels in the neck compressed become insensible. But
sleep supervenes when such incapacity of exercise has neither arisen in
some casual organ of sense, nor from some chance cause, but when, as has
been just stated, it has its seat in the primary organ with which one perceives
objects in general. For when this has become powerless all the other sensory
organs also must lack power to perceive; but when one of them has become
powerless, it is not necessary for this also to lose its
power.

We must next state the cause to which it is due, and its quality
as an affection. Now, since there are several types of cause (for we assign
equally the 'final', the 'efficient', the 'material', and the 'formal'
as causes), in the first place, then, as we assert that Nature operates
for the sake of an end, and that this end is a good; and that to every
creature which is endowed by nature with the power to move, but cannot
with pleasure to itself move always and continuously, rest is necessary
and beneficial; and since, taught by experience, men apply to sleep this
metaphorical term, calling it a 'rest' [from the strain of movement implied
in sense-perception]: we conclude that its end is the conservation of animals.
But the waking state is for an animal its highest end, since the exercise
of sense-perception or of thought is the highest end for all beings to
which either of these appertains; inasmuch as these are best, and the highest
end is what is best: whence it follows that sleep belongs of necessity
to each animal. I use the term 'necessity' in its conditional sense, meaning
that if an animal is to exist and have its own proper nature, it must have
certain endowments; and, if these are to belong to it, certain others likewise
must belong to it as their condition.

The next question to be discussed is that of the kind of movement
or action, taking place within their bodies, from which the affection of
waking or sleeping arises in animals. Now, we must assume that the causes
of this affection in all other animals are identical with, or analogous
to, those which operate in sanguineous animals; and that the causes operating
in sanguineous animals generally are identical with those operating in
man. Hence we must consider the entire subject in the light of these instances
[afforded by sanguineous animals, especially man]. Now, it has been definitely
settled already in another work that sense-perception in animals originates
ill the same part of the organism in which movement originates. This locus
of origination is one of three determinate loci, viz. that which lies midway
between the head and the abdomen. This is sanguineous animals is the region
of the heart; for all sanguineous animals have a heart; and from this it
is that both motion and the controlling sense-perception originate. Now,
as regards movement, it is obvious that that of breathing and of the cooling
process generally takes its rise there; and it is with a view to the conservation
of the [due amount of] heat in this part that nature has formed as she
has both the animals which respire, and those which cool themselves by
moisture. Of this [cooling process] per se we shall treat hereafter. In
bloodless animals, and insects, and such as do not respire, the 'connatural
spirit' is seen alternately puffed up and subsiding in the part which is
in them analogous [to the region of the heart in sanguineous animals].
This is clearly observable in the holoptera [insects with undivided wings]
as wasps and bees; also in flies and such creatures. And since to move
anything, or do anything, is impossible without strength, and holding the
breath produces strength-in creatures which inhale, the holding of that
breath which comes from without, but, in creatures which do not respire,
of that which is connatural (which explains why winged insects of the class
holoptera, when they move, are perceived to make a humming noise, due to
the friction of the connatural spirit colliding with the diaphragm); and
since movement is, in every animal, attended with some sense-perception,
either internal or external, in the primary organ of sense, [we conclude]
accordingly that if sleeping and waking are affections of this organ, the
place in which, or the organ in which, sleep and waking originate, is self-evident
[being that in which movement and sense-perception originate, viz. the
heart].

Some persons move in their sleep, and perform many acts like waking
acts, but not without a phantasm or an exercise of sense-perception; for
a dream is in a certain way a sense-impression. But of them we have to
speak later on. Why it is that persons when aroused remember their dreams,
but do not remember these acts which are like waking acts, has been already
explained in the work 'Of Problems'.

Part 3

The point for consideration next in order to the preceding is:-What
are the processes in which the affection of waking and sleeping originates,
and whence do they arise? Now, since it is when it has sense-perception
that an animal must first take food and receive growth, and in all cases
food in its ultimate form is, in sanguineous animals, the natural substance
blood, or, in bloodless animals, that which is analogous to this; and since
the veins are the place of the blood, while the origin of these is the
heart-an assertion which is proved by anatomy-it is manifest that, when
the external nutriment enters the parts fitted for its reception, the evaporation
arising from it enters into the veins, and there, undergoing a change,
is converted into blood, and makes its way to their source [the heart].
We have treated of all this when discussing the subject of nutrition, but
must here recapitulate what was there said, in order that we may obtain
a scientific view of the beginnings of the process, and come to know what
exactly happens to the primary organ of sense-perception to account for
the occurrence of waking and sleep. For sleep, as has been shown, is not
any given impotence of the perceptive faculty; for unconsciousness, a certain
form of asphyxia, and swooning, all produce such impotence. Moreover it
is an established fact that some persons in a profound trance have still
had the imaginative faculty in play. This last point, indeed, gives rise
to a difficulty; for if it is conceivable that one who had swooned should
in this state fall asleep, the phantasm also which then presented itself
to his mind might be regarded as a dream. Persons, too, who have fallen
into a deep trance, and have come to be regarded as dead, say many things
while in this condition. The same view, however, is to be taken of all
these cases, [i.e. that they are not cases of sleeping or
dreaming].

As we observed above, sleep is not co-extensive with any and every
impotence of the perceptive faculty, but this affection is one which arises
from the evaporation attendant upon the process of nutrition. The matter
evaporated must be driven onwards to a certain point, then turn back, and
change its current to and fro, like a tide-race in a narrow strait. Now,
in every animal the hot naturally tends to move [and carry other things]
upwards, but when it has reached the parts above [becoming cool], it turns
back again, and moves downwards in a mass. This explains why fits of drowsiness
are especially apt to come on after meals; for the matter, both the liquid
and the corporeal, which is borne upwards in a mass, is then of considerable
quantity. When, therefore, this comes to a stand it weighs a person down
and causes him to nod, but when it has actually sunk downwards, and by
its return has repulsed the hot, sleep comes on, and the animal so affected
is presently asleep. A confirmation of this appears from considering the
things which induce sleep; they all, whether potable or edible, for instance
poppy, mandragora, wine, darnel, produce a heaviness in the head; and persons
borne down [by sleepiness] and nodding [drowsily] all seem affected in
this way, i.e. they are unable to lift up the head or the eye-lids. And
it is after meals especially that sleep comes on like this, for the evaporation
from the foods eaten is then copious. It also follows certain forms of
fatigue; for fatigue operates as a solvent, and the dissolved matter acts,
if not cold, like food prior to digestion. Moreover, some kinds of illness
have this same effect; those arising from moist and hot secretions, as
happens with fever-patients and in cases of lethargy. Extreme youth also
has this effect; infants, for example, sleep a great deal, because of the
food being all borne upwards-a mark whereof appears in the disproportionately
large size of the upper parts compared with the lower during infancy, which
is due to the fact that growth predominates in the direction of the former.
Hence also they are subject to epileptic seizures; for sleep is like epilepsy,
and, in a sense, actually is a seizure of this sort. Accordingly, the beginning
of this malady takes place with many during sleep, and their subsequent
habitual seizures occur in sleep, not in waking hours. For when the spirit
[evaporation] moves upwards in a volume, on its return downwards it distends
the veins, and forcibly compresses the passage through which respiration
is effected. This explains why wines are not good for infants or for wet
nurses (for it makes no difference, doubtless, whether the infants themselves,
or their nurses, drink them), but such persons should drink them [if at
all] diluted with water and in small quantity. For wine is spirituous,
and of all wines the dark more so than any other. The upper parts, in infants,
are so filled with nutriment that within five months after
birththey do not even turn the neck [sc. to raise thehead]; for in them, as
in persons deeply intoxicated, there is ever a large quantity of moisture
ascending. It is reasonable, too, to think that this affection is the cause
of the embryo's remaining at rest in the womb at first. Also, as a general
rule, persons whose veins are inconspicuous, as well as those who are dwarf-like,
or have abnormally large heads, are addicted to sleep. For in the former
the veins are narrow, so that it is not easy for the moisture to flow down
through them; while in the case of dwarfs and those whose heads are abnormally
large, the impetus of the evaporation upwards is excessive. Those [on the
contrary] whose veins are large are, thanks to the easy flow through the
veins, not addicted to sleep, unless, indeed, they labour under some other
affection which counteracts [this easy flow]. Nor are the 'atrabilious'
addicted to sleep, for in them the inward region is cooled so that the
quantity of evaporation in their case is not great. For this reason they
have large appetites, though spare and lean; for their bodily condition
is as if they derived no benefit from what they eat. The dark bile, too,
being itself naturally cold, cools also the nutrient tract, and the other
parts wheresoever such secretion is potentially present i.e. tends
to be formed.

Hence it is plain from what has been said that sleep is a sort
of concentration, or natural recoil, of the hot matter inwards [towards
its centre], due to the cause above mentioned. Hence restless movement
is a marked feature in the case of a person when drowsy. But where it [the
heat in the upper and outer parts] begins to fail, he grows cool, and owing
to this cooling process his eye-lids droop. Accordingly [in sleep] the
upper and outward parts are cool, but the inward and lower, i.e. the parts
at the feet and in the interior of the body, are hot.

Yet one might found a difficulty on the facts that sleep is most
oppressive in its onset after meals, and that wine, and other such things,
though they possess heating properties, are productive of sleep, for it
is not probable that sleep should be a process of cooling while the things
that cause sleeping are themselves hot. Is the explanation of this, then,
to be found in the fact that, as the stomach when empty is hot, while replenishment
cools it by the movement it occasions, so the passages and tracts in the
head are cooled as the 'evaporation' ascends thither? Or, as those who
have hot water poured on them feel a sudden shiver of cold, just so in
the case before us, may it be that, when the hot substance ascends, the
cold rallying to meet it cools [the aforesaid parts] deprives their native
heat of all its power, and compels it to retire? Moreover, when much food
is taken, which [i.e. the nutrient evaporation from which] the hot substance
carries upwards, this latter, like a fire when fresh logs are laid upon
it, is itself cooled, until the food has been digested.

For, as has been observed elsewhere, sleep comes on when the corporeal
element [in the 'evaporation'] conveyed upwards by the hot, along the veins,
to the head. But when that which has been thus carried up can no longer
ascend, but is too great in quantity [to do so], it forces the hot back
again and flows downwards. Hence it is that men sink down [as they do in
sleep] when the heat which tends to keep them erect (man alone, among animals,
being naturally erect) is withdrawn; and this, when it befalls them, causes
unconsciousness, and afterwards phantasy.

Or are the solutions thus proposed barely conceivable accounts
of the refrigeration which takes place, while, as a matter of fact, the
region of the brain is, as stated elsewhere, the main determinant of the
matter? For the brain, or in creatures without a brain that which corresponds
to it, is of all parts of the body the coolest. Therefore, as moisture
turned into vapour by the sun's heat is, when it has ascended to the upper
regions, cooled by the coldness of the latter, and becoming condensed,
is carried downwards, and turned into water once more; just so the excrementitious
evaporation, when carried up by the heat to the region of the brain, is
condensed into a 'phlegm' (which explains why catarrhs are seen to proceed
from the head); while that evaporation which is nutrient and not unwholesome,
becoming condensed, descends and cools the hot. The tenuity or narrowness
of the veins about the brain itself contributes to its being kept cool,
and to its not readily admitting the evaporation. This, then, is a sufficient
explanation of the cooling which takes place, despite the fact that the
evaporation is exceedingly hot.

A person awakes from sleep when digestion is completed: when the
heat, which had been previously forced together in large quantity within
a small compass from out the surrounding part, has once more prevailed,
and when a separation has been effected between the more corporeal and
the purer blood. The finest and purest blood is that contained in the head,
while the thickest and most turbid is that in the lower parts. The source
of all the blood is, as has been stated both here and elsewhere, the heart.
Now of the chambers in the heart the central communicates with each of
the two others. Each of the latter again acts as receiver from each, respectively,
of the two vessels, called the 'great' and the 'aorta'. It is in the central
chamber that the [above-mentioned] separation takes place. To go into these
matters in detail would, however, be more properly the business of a different
treatise from the present. Owing to the fact that the blood formed after
the assimilation of food is especially in need of separation, sleep [then
especially] occurs [and lasts] until the purest part of this blood has
been separated off into the upper parts of the body, and the most turbid
into the lower parts. When this has taken place animals awake from sleep,
being released from the heaviness consequent on taking food. We have now
stated the cause of sleeping, viz. that it consists in the recoil by the
corporeal element, upborne by the connatural heat, in a mass upon the primary
sense-organ; we have also stated what sleep is, having shown that it is
a seizure of the primary sense-organ, rendering it unable to actualize
its powers; arising of necessity (for it is impossible for an animal to
exist if the conditions which render it an animal be not fulfilled), i.e.
for the sake of its conservation; since remission of movement tends to
the conservation of animals.